Silicic gases as referred to herein are silicon compounds or mixtures of silicon compounds which under the conditions according to the invention can be decomposed in the gaseous phase depositing silicon. Silicon-free gases in the meaning of this invention are gases which do not contain any silicon compounds.
For the production of elemental silicon with a purity allowing its being used in the photovoltaic area and/or in semi-conductor technology, methods of thermal decomposition of volatile silicon compounds are known. Such thermal decomposition can be carried out, for example, in fluidized-bed reactors in that small silicon particles are provided which are then fluidized by an appropriate silicic gas or gas mixture flowing into the reactor, whereby the gases in the gas mixture can be silicic, but also silicon-free gases. Ideally, the thermal decomposition of such volatile silicon compounds shall occur exclusively on the surface of the small silicon particles are provided. The said small silicon particles provided, hereinafter referred to as nucleus particles, form a large area for the separation of silicon within the reactor. Particles that have grown to a sufficient size are removed from the reactor and new nucleus particles are introduced.
In addition to the separation of silicon on the particles during the decomposition of the gas containing silicon also silicon dust is produced which is difficult to handle and can be easily contaminated due to its large surface. Further the produced silicon dust contains significant amounts of hydrogen obstructing the subsequent melting on process. For this reason the formation of dust is undesired. Silicon dust in this context refers to silicon particles with a diameter of particles of up to approx. 25 μm.
It is known that in the case of decomposition of gas containing silicon in fluidized-bed reactors a major part of the dust is formed in a homogeneous reaction. This reaction occurs predominantly in the so-called bubble phase. The silicon formed in such phase by decomposition of gases containing silicon does not result in an expansion of provided nucleus particles by separation on said nucleus particles (Chemical Vapour Deposition CVD), but forms dust that is carried out from the reactor.
U.S. Pat. No. 4,818,495 A discloses a method for the production of silicon particles by thermal decomposition of a gas containing silicon in a fluidized bed. This is where silicon nucleus particles are added, on which deposits the silicon that has formed by decomposition.
On principle, so-called bubble breakers are suitable for the reduction of gas bubbles in a fluidized bed thus reducing the bubble phase. Different apparatuses with bubble-breaking function are known, e.g. vertical and horizontal gas distribution plates, geometrical constructions, such as tube combinations and three-dimensional grids and packings (tower packing, wires etc.). Such apparatuses have the disadvantage in fluidized beds and particularly under the aggressive reaction conditions prevailing at thermal decomposition of a gas containing silicon that the intensive radial and axial mixing can be diminished and that a strong erosion of the built-in elements must be expected. Such serious mechanical strain limits the number of materials suitable for the said built-in elements. Apart from this it must be ensured, particularly with regard to the high purity required for the desired use of the produced silicon in the semi-conductor or photovoltaic area, that the bubble breakers do not carry any contamination into the silicon. A silicon separation on the bubble breaker elements leads to internal overgrowing of the reactor. The construction and the scale-up of such bubble breakers are difficult.